1. Field of the Invention
The present invention relates to apparatuses and methods for applying material to a substrate and, for preventing applied material from adhering to processing equipment.
2. State of the art
In recent years, workers in the battery art have begun to understand and recognize the advantages of "laminate" batteries including solid polymer electrolytes and sheet-like anodes and cathodes. The advantages include lower battery weights than conventional batteries that employ liquid electrolytes, longer service life, relatively high power densities, relatively high specific energies, and the elimination of the danger associated with batteries containing spillable liquid electrolytes such as acids.
Laminate batteries using polymer electrolytes have been developed which possess good performance characteristics. For example, in U.S. Pat. No. 4,925,751, a laminate battery in which a cathode material formed from a mixture of an active cathodic material (preferably the vanadium oxide compound V.sub.6 O.sub.13), a conductive filler material (preferably carbon particles or filaments), and an ionically conductive polymer electrolyte material is laminated on a conductive substrate material such as a continuous sheet of aluminum or nickel. A layer of polymer electrolyte material is laminated over the laminated layer of cathode material, and an anode material is applied over the laminated layer of polymer electrolyte material.
The cathode material typically has a viscosity on the order of about 900,000 centipoise at 200 sec.sup.-1. The application of the cathode material onto the substrate is generally performed through an extrusion process in which a bead of cathode material is extruded onto the substrate, and the substrate and the extruded cathode material are compressed between a pair of polished pressing rollers to form a cathode material layer. Formation of an even thickness layer is difficult, in part because of the tendency of the uncured cathode material to adhere to any surface with which it comes into contact, particularly the rollers for forming the cathode layer on the substrate. Further, in handling the substrate coated with uncured cathode material at subsequent processing points, such as at cathode layer smoothing devices, the cathode material tends to adhere to the subsequent processing equipment.
In attempting to minimize problems associated with cathode material adhering to processing equipment, various solutions have been proposed. Typically, the cathode material layer formed on a substrate is cured after formation of the cathode material layer by the pressing rollers. While curing the cathode material layer after its formation by the pressing rollers reduces adherence of cathode material to subsequent processing equipment, it does not prevent cathode material from adhering to the pressing rollers for forming the cathode material layer, which must be positioned before any curing apparatus. Further, when the cathode material layer is cured prior to application of the electrolyte material layer, it is necessary to provide a second curing step for curing the electrolyte material layer. It is, therefore, desirable to provide an apparatus and method for applying cathode material to a substrate in which cathode material is prevented from adhering to pressing rollers and in which only one curing step is necessitated in a process for forming a laminate including a cathode material layer and an electrolyte material layer.
Copending U.S. patent application Ser. No. 08/078,940 (corresponding to Valence Technology, Inc., I.D. No. 1107) describes one technique for minimizing problems associated with cathode material adhering to surfaces. A web of material, such as Mylar, is wound around the one of the pressing rollers that presses against the bead of uncured cathode material. When cathode material is extruded onto the substrate, the cathode material is squeezed by the pressing rollers between the substrate and the continuous web, rather than being in direct contact with one of the rollers. The web, or "release layer", is later peeled away from the cathode material layer. This method and apparatus prevents buildup of cathode material on the pressing rollers, and the web is preferably a highly smooth material which has a lesser tendency to permit cathode material to adhere to it than the polished pressing roller. However, it has been observed that the cathode material nonetheless tends to adhere to the release layer when it is peeled away and thus causes an imperfectly smooth cathode material layer. Further, the release layer has been known to carry impurities that affect the quality of the cathode material layer.
Copending U.S. patent application Ser. No. 07/968,370 (corresponding to Valence Technology, Inc., I.D. No. 1106) describes another technique for preventing adherence of cathode material to processing equipment including pressing rollers. Chilled rollers are used in forming and smoothing the cathode material layer. The temperature of the rollers is such that the cathode material tends to become set or cast, although not cured, so that it will not adhere to the chilled rollers. Unfortunately, this technique is also only partially successful, in that there is still some tendency for the cathode material to adhere to the chilled rollers. Accordingly, it is desirable to provide a method and apparatus for forming and processing a web coated with a cathode material layer which minimizes the tendency of cathode material to adhere to the layer forming and processing equipment.